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Lecture 5
Chapter 11
Unicast Routing
Protocols
(RIP, OSPF, BGP)
(How the routers’ tables are filled in)
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Routing Protocols
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At this stage, we understand how a router uses a routing table in
making a next hop decision
However, what dictates HOW the routing tables are filled in ?
Tables are concerned about the next hop only
What’s responsible for looking across the entire path or route –
what makes the decision of the best route ?
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Routing Protocols and Algorithms are used
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Routing protocols allow routers to share info with one another
dynamically - as the Internet makes changes, the routing protocols
allow routers to inform other routers
Routers communicate to their neighboring routers - gossip
Routing protocols implement the procedures for combining info
received from other routers
Routing Algorithms – decision making analysis – the “brains” –
using the info provided
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•
•
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Autonomous systems
• Because the Internet is so large, one protocol cannot handle all of the updating
of tables – create groups and networks and routers called Autonomous Systems
• Routing within the autonomous system is called “interior routing”
• Routing between the autonomous systems is called “exterior routing”
NOTE: different
interior routing
protocols can be used
for each autonomous
systems HOWEVER,
only one exterior
routing protocol is
used
R1, R2, R3 and R4 use an interior and exterior routing protocol – all other routers
only use an interior routing protocol
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Tx
How does it work ?
• A certain “cost” or
“metric” is assigned
each network
Rx
• In figuring out the
best route from Tx to
Rx, the set of
networks with the
smallest sum is
chosen
• More generically, the
set of networks best
• If #hops was the metric, we would want to traverse
meeting the
the least number of networks in going from Tx to Rx
“metric’s” objective
is chosen
• If max throughput was the metric, a fiber optic
network would have a better metric than a coaxial
network.
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Explain routing using your street/highway analogy
• Autonomous systems
• Default routing
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Popular Unicast Routing protocols
• RIP – Routing Information Protocol – treats each network the same (assigns
the same cost for each network)
• OSPF – Open Shortest Path First protocol – assigns a cost for passing
through a network based on the type of service required – routes through the
network can have different cost – each router would have several tables
• BGP – Border Gateway Protocol – is an exterior routing protocol that uses a
policy that defines what paths should be chosen
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RIP Algorithm
Distance Vector Algorithm built from Bellman-Ford Algorithm
Recall: each router sends message to it’s neighbor
For the router receiving a RIP response
1st – add one to hop count for each destination advertised
2nd – repeat the following steps for each advertised destination
1. If destination is not in table
add destination to table
2. Else if destination is in table
1. If next-hop field is the same
replace entry in table with advertised one
2. Else next-hop different
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replace entry if advertised hop count is less
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Example RIP Algorithm
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Router receives RIP message for some router C
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The RIP message list destination networks, corresponding hop
count and next hop (not listed in diagram)
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1st step: increment hop count
• Net1: no news, don’t change
• Net2: same next hop, so replace 2 with 5
• Net 3: new router, so add
• Net 6: different next hop, new hop count
less, so replace
• Net 8: different next hop, new hop count
the same, don’t change
• Net 9: different next hop, new hop count
larger, do not change
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Initial routing tables in a small
autonomous system
Initial tables are created from config file (and hop counts are set to 1) – next
hop fields are empty initially because all networks are directly connected
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Final routing tables for the previous figure
For example, suppose packet hitting Router A first had a destination of Net 66 ?
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RIP message format
• Command – 8-bit field specifying the type of message: response (2) or request (1)
• Version – 8-bit field specifying RIP version
• Family – 16-bit specifying protocol family (TCP/IP=2)
• Network Address – address of the destination network
• Distance – 32-bit field defining the hop count from advertising router to destination network
NOTE: Request can be issued by a newly added router or by a router seeking certain info
NOTE: 2 response types: Solicited – response to request, Unsolicited – periodic updates
Gray fields repeated for each destination network
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Example 1
What is the periodic response sent by router
R1 in the figure below. Assume R1 knows
about the whole autonomous system.
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Solution
R1 can advertise three networks 144.2.7.0, 144.2.9.0, and 144.2.12.0.
The periodic response (update packet) is shown below
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RIP timers
• Periodic Timer – each router has timer set to 25-35 secs and when the timer counts
down, an update message is sent
• Expiration Timer – governs the validity of the next-hop – when router receives next-hop
update, timer is set to 180 sec. If there is a problem and the router doesn’t receive it’s 30
sec update, the route info expires (invalid) after the 180 sec count down – then the hop
count is set to 16 (infinity)
• Garbage Collection Timer – once the route expires, this timer is set to 120 sec and counts
downs – allows neighbors time to become aware of invalidity – after count down, info is
purged
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Problems with RIP
• Slow Convergence – the time it takes a change in
the Internet to propagate through the rest of the
Internet – recall the periodic updates with neighbors.
DEPENDING ON THE DATA RATE, millions or
billions of bits could be sent in that time – therefore
possibly lost
nx15s
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RIP Problem - Instability
• RA and RB has hop counts 1 and
2, respectively for Net1
• Net1 goes down – RA can update
fast due to direct connection –
sets hop count to 16
• RA has to wait 30 sec to update
RB (this is the problem)
• In meanwhile, RB sends update
to RA with hop count 2 for Net1
(incremented to 3)
• Now when RA finally send the
update to RB, it sends a hop
count of 3 (incremented to 4) –
RA thinks it’s another route to
Net1
• This INSTABILITY (back-andforth) continues until both set hop
count to 16
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Popular Unicast Routing protocols
• RIP – Routing Information Protocol – treats each network the same (assigns
the same cost for each network)
• OSPF – Open Shortest Path First protocol – assigns a cost for passing
through a network based on the type of service required – routes through the
network can have different cost – each router would have several tables
• BGP – Border Gateway Protocol – is an exterior routing protocol that uses a
policy that defines what paths should be chosen
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OSPF: Open Shortest Path First
• Similar to RIP however, divide autonomous system into areas
• Routers with in an area floods the area with routing info – router sends to all it’s neighbors
and each neighbor sends to all it’s neighbors and etc..
• At the border of an area, special routers called area border routers are used to (1)
summarize info about an area and (2) send info amongst areas
• A special area called the backbone is used to tie together all of the areas – backbone is
primary area and all other areas are secondary areas – backbone area uses backbone
routers
• Note: backbone router can also be an area border router
• Each area has an ID (backbone’s Id is 0)
Explain Current Real World Practices relating to this topic
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Recall OSPF: Open Shortest Path First
• The OSPF is similar to RIP however, it allows the admin the
ability to assign a cost or metric to each route.
• The metric can be based on a type of service (ie . Min delay,
max throughput, etc..)
• For OSPF, a router will have multiple routing tables – one
for each TOS
• Unlike RIP, sharing or updating is done when there is a
change (not periodically)
• For OSPF, the objective is for the routers to contain the full
picture or topology of the Internet – by having this, the
router can figure out the “shortest path” or “least cost” route
between itself and each network
• To do this, the Internet is represented by a graph – set of
edges and nodes
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Defining edges or connections/links
Point-to-point link
Virtual link
Direct connection between two
routers, no IP address needed
Bi-directional Edge
When link between two routers are
broken, admin creates a new route
across multiple routers
Transient link
Stub link
Represents
the network
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Connects to only one router – packets
enter and leave through this same
router
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Graphical representation of an internet
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OSPF uses Dijkstra’s Shortest Path Algorithm
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Dijkstra’s Algorithm Cont..
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Dijkstra’s Algorithm Continued
At this point, we would have the least cost path from A to all other nodes
Note: to find the least cost paths from E to all other nodes, Disjkstra’s algorithm has to run
again
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Answer in Hyphen Format
A-D
A-D-N3
A-D-N3-F
A-N1-B-E
A-N1-B-E-N4
A-N1-C
A-D-N3-F-N5
A-N1-C-N2
A-N1
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A-N1-B
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A re-occurring theme
How do the OSPF packets travel across the
network(s) ??????????????
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OSPF packets are
encapsulated in IP datagrams.
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BGP: Border Gateway Protocol
BPG is an interautonomous
system routing
protocol that
makes use of path
vector routing
For BPG, each routing entry contains (1) destination network, (2)
next router (hop) and (3) entire path to reach destination
Path is an ordered list of
autonomous systems that
the packet should travel
through to make it to the
destination
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